Effect of concentration on reaction rate

Question 1

Identify the pale yellow precipitate that obscures the cross on the sheet of paper

Answer 1

Colloidal sulfur

Question 2

Describe the procedure for preparing the 0.08M solution of sodium thiosulfate from the 0.10M solution

Answer 2

0.1 = 100 cm3
Add 80cm3 of solution using burette
Make up until 100cm3 with deionised water

Question 3

Explain why the reciprocal of time [1/time] is used as a measure of the initial rate of the reaction?

Answer 3

Rate of reaction is inversely promotional to the time. When time increases, there is a decrease in rate

Question 4

Why are the concentration and the volume of HCL solution kept constant?

Answer 4

So that there is only one variable that changes - ensures fair experiment

Question 5

Outline the experimental procedures you would follow in order to show the effect of concentration on reaction rate

Answer 5

Place a conical flask on a piece of paper marked with an X
Place fixed volume of a given concentration of thiosulfate in the flask
Add fixed volume of HCL to the flask and note the time taken for sulfur to obscure x, swirl conical flask, reaction is finished
Rate of reaction for concentration is inversely promotional to the time taken
Repeat the experiment with different concentrations

Question 6

State the relationship between the rate of the reaction and the concentration of sodium thiosulfate

Answer 6

The rate of reaction is directly propotional to the concentration of the thiosulfate.

Question 7

Explain why the rate of reaction increases with thiosulfate concentration

Answer 7

Increased concentration of thio means more molecules in a given molecule therefore more successful collisions and thus increase in rate of reaction

Question 8

Describe a method you could use to determine when the same mass of sulfur had been formed in each run

Answer 8

X on a piece of Paper which ensures a fair test

When it’s no longer seen, sulfur has precipitated and shows same amount

Question 9

If you were given sodium thiosulfate solutions of the following concentrations : 0.04M, 0.08M, 0.12M and 0.16M, describe how you would show that the rate of reaction is directly proportional to the concentration of sodium thiosulfate

Answer 9

• do method
• find 1/t
• plot vs concentration

Question 10

What is meant by the term concentration?

Answer 10

Amount of solute that is dissolved in a given volume of solution

Question 11

What is the purpose of placing a mark on the paper?

Answer 11

The X serves as a standard marker that helps us determine the stage at which the same amount of sulfur has precipitated for each experiment and ensures it is a ‘fair test’

Question 12

Describe the appearance of the suspension formed

Answer 12

Cloudy (milky) yellow colour

Question 13

Apart from the element causing the yellow suspension, what other products are formed in this reaction?

Answer 13

Sodium chloride, sulfur dioxide and water

Question 14

Why must the flask and graduated cylinder be rinsed out after each reading?

Answer 14

To avoid contamination of the next set of solutions from the previous experiment i.e ensure droplets of previous solutions do not alter concentrations

Question 15

What happened to the rate of reaction as the concentration was decreased?

Answer 15

The rate also decreased

Question 16

Time and rate are said to be inversely proportional, explain what this means

Answer 16

As time is increased, the rate is decreased i.e the longer the time it takes the cross to be obscured, the slower is the rate of reaction

Question 17

What is the relationship between time and rate?

Answer 17

Inversely proportional, as time increases the rate is decreased

Question 18

Give the balanced equation for the reaction

Answer 18

2HCL + Na2S2O3 -> 2Nacl + SO2 + S(down) +H20

Question 19

Describe a procedure for diluting the 0.20 M sodium thiosulfate solution to give 50 cm3 of a 0.12 M solution.

Answer 19

(.20/.12= 5/3. 50 / 5/3 = 30)

Measure 30 cm3 of the 0.20 M thiosulfate solution dilute (make up) to 50 cm3 using deionised water